Bile cast nephropathy is one cause of acute or chronic kidney injury in patients with liver disease (especially obstructive cholestasis). Figure 1 shows two paraffin blocks containing kidney biopsies from two different patients. The upper block shows the typical appearance of a paraffin-embedded core biopsy of kidney tissue. The tissue in the lower block (indicated by the arrow) shows prominent green discoloration and was taken from a patient with hyperbilirubinemia due to underlying liver disease. Figure 2 shows the corresponding light microscopic findings, which include acute tubular injury associated with numerous green-brown casts. Histochemical staining confirmed the presence of bile (stain not shown). The casts are thought to cause kidney injury by distal nephron obstruction and direct toxicity to the tubular epithelium by bilirubin and bile acids.